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The hack is controlled by following environment variables:
QEMU_VGA_RETRACE_HACK

**********************************************************************
If numeric value of QEMU_VGA_RETRACE_HACK is 1:
"Precise" retrace will be emulated, that is VGA emulation will try to
return the values of 'Input Status #1 Register' as close to the real
VGA as possible.

**********************************************************************
If numeric value of QEMU_VGA_RETRACE_HACK is 2:

Following variables will be used
QEMU_VGA_RETRACE_PERIOD
QEMU_VGA_RETRACE_FREQUENCY

Frequency (QEMU_VGA_RETRACE_FREQUENCY) is specified in hertz.
Period (QEMU_VGA_RETRACE_PERIOD) is specified in hertz * 1000000.

Default for QEMU_VGA_RETRACE_FREQUENCY(when not set is 70).
Default for QEMU_VGA_RETRACE_PERIOD(when not set is 0).


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PIT mode-3 hack is controlled by following environment variable:
QEMU_MODE3_VAL
(optionally QEMU_VGA_RETRACE_FREQUENCY)

If it is positive then program reading from PIT in mode-3 will always
get this value.

If it is -1 then value of QEMU_VGA_RETRACE_FREQUENCY is read from the
environment (70 is used when this environment variable is not set) and
is converted into PIT units and reading from PIT in mode-3 will always
get this value.

If it is negative then it's absolute value (in hertz) is converted
into PIT units and reading from PIT in mode-3 will always get this
value.


======================================================================
Example usage:

a. prefered

$ export QEMU_VGA_RETRACE_HACK=1
$ qemu ...

b. if you are desperate

$ export QEMU_VGA_RETRACE_HACK=2
$ export QEMU_VGA_RETRACE_PERIOD=1000
$ export QEMU_VGA_RETRACE_FREQUENCY=30
$ export QEMU_PIT_MODE3_VAL=-1
$ qemu ...